1. Field of the Invention
This invention relates generally to filter assemblies and to improved methods for the manufacture of such filter assemblies. More particularly, this invention relates to filter assemblies having a pleated filter medium mounted within or attached to a support frame surrounding the filter medium and to improved methods utilizing a welding process to seal the edges of the filter medium to the frame around the inside perimeter thereof to form a final filter assembly exhibiting an essentially leak-proof seal.
2. Description of the Prior Art
The construction of filter assemblies containing pleated filter media enclosed within a support frame presents difficult problems in bonding the periphery of the medium to the frame. In order to obtain a useful and reliable filter assembly, it is essential that a strong, leak-proof seal be created at the filter/frame junction.
A number of techniques are currently in use for sealing a filter medium to a support frame. Most traditional methods in use confine the ends of the pleats to a rectangularly recessed channel located within the support frame. An adhesive such as urethane, hot melt, epoxy or some other adhesive is applied within the channel providing a bond between the media edge and the support frame inner perimeter. However, such adhesives are labor intensive to apply and could involve time consuming cleaning procedures. In addition, the filters thus produced may be undesirable from an environmental standpoint and the resulting filter assembly may be limited to relatively low temperature applications. Moreover, as the ends of the pleated filter medium are free to float within the frame, an uneven distribution of pleats may occur during the production process.
Some manufacturers, particularly those producing vehicular filters employ a process described as "insert molding". In this procedure, the filter medium is pleated as required, cut to size, and then placed into an injection mold tool. The plastic frame is then made by injection molding and the filter media ends are trapped and sealed within the plastic of the frame. Although this molding technique is expensive due to high tooling costs, it provides a method of manufacture which is capable of high volume production.
Various welding and swaging techniques have been used in the production of respiratory canister filter assemblies as for example, to bond fines filters and/or retainers to the bottom or side wall of a metallic canister housing. In addition, many types of disposable respirator masks use ultrasonic welding techniques for sealing the filter mask unit.
U.S. Pat. No. 3,815,754 discloses the application of ultrasonic welding to the production of plastic filter assemblies used for blood filtration. In this patent ultrasonic welding is disclosed for sealing the filter to the housing and for sealing the housing elements to each other to prevent fluid leakage to the outside of the filter assembly.
While such applications of ultrasonic welding are known within the filter manufacturing industry, such techniques have not been used for permanently affixing a pleated filter assembly to a support frame, to create a leak-proof seal along the perimeter edge of each individual pleat present in the filter assembly. Certainly, in filtering assemblies which require more than two pleats per inch to ensure sufficient filtration surface area, the ability to direct bonding energy to the desired edge surface becomes increasingly more difficult. Prior to the instant invention, it was generally believed that welding applications, and particularly ultrasonic welding, would not be capable of forming an appropriately uniform leak-proof seal. Hence, such welding techniques were not thought to be applicable for use with the densely pleated filters or filter pack assemblies in use today.